Like many veteran geeks I have a different iCloud ID and Apple store ID. This used to be a supported configuration. In my case it was essential because of some complicated history with Apple’s .mac precursor to MobileMe and iCloud. (In an unrelated matter I have another 3-4 Apple IDs that aren’t connected to anything but, depending on the vagaries of Apple’s hacked together legacy databases, sometimes pickup Apple Store hardware purchases.)
This is what Apple’s support document says now (emphases mine) …
… We recommend that you use the same Apple ID for all Apple services on your device—including the iTunes & App Stores and iCloud …
… If you have multiple Apple IDs, you can’t merge them …
I went looking for this document because I think iBooks.app doesn’t work properly with an iCloud ID that’s different from the Apple Store ID that can be used to purchase iBooks. It looks like this will be a trend.
Note what Apple says here. Your Apple Store ID and iCloud ID should be the same. You also can’t merge them . So you either need to abandon all your Apple Store purchases or your iCloud storage purchase.
Anyone remember when Cook promised to fix Apple’s original sin of botched identity management system? Apparently the problem is harder than building spaceship headquarters.
Apple should bite the bullet and come up with a process to merge Apple IDs. I fear they aren’t going to bother though. I really miss class action lawsuits.
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 There is a possible workaround. You may be able to use your iCloud ID as an Apple Store ID and then make it a family member of the original Apple Store ID. This will run into rules about changing device Store IDs and constraints on family member size as well as issues with the total number of devices that are part of a family (10). It isn’t an official workaround and I suspect it has irreversible problems of its own.